Low-level pure-tone masking : a comparison of tuning curves obtained with simultaneous and forward masking

Simultaneous and forward pure-tone masking are compared, using a fixed-level probe of 20-ms and a 200-ms masker. For a 1-kHz probe of 30 dB SPL the required masker level Lm is measured as a function of the time interval ?t between masker offset and probe onset. When masker and probe have equal frequencies a monotonic relationship is found for phase ?/2 but not for phase 0. When the masker frequency fm is 50 or 100 Hz below the probe frequency fp a nonmonotony is found, with a minimum at ?t=0, the transition between simultaneous and forward masking. When fm is 50 or 100 Hz above fp, however, the relationship of Lm to ?t is monotonic. In the case of simultaneous masking the iso-Lp curves, which give Lm as a function of fm, show a typical asymmetry around fm=fp, leading to the positive shift of the maximum masking frequency MMF previously reported for stationary pure-tone maskers. In the case of forward masking, however, this asymmetry ceases to exist. We conclude that simultaneity of probe and masker is a necessary condition for the occurrence of a low-level positive MMF shift. The results are discussed in the light of psychoacoustical and neurophysiological data on two-tone suppression. A possible interpretation of the nonmonotony and of the positive MMF shift is suggested in terms of the physiological asymmetry in two-tone suppression

A mixed-excitation vocoder based on exact analysis of harmonic components

A new analysis-synthesis algorithm has been developed for high quality diphone speech synthesis, based on accurate measurement of the mixture of periodic and noise information in speech. Input speech is analysed pitch synchronously, using a refined pitch estimation by means of 'first-harmonic filtering'. Accurate pitch forms the basis for a Discrete Fourier Transform (OFT), providing exact amplitudes and phases of all harmonics. For each harmonic a 'factor of noisiness' is calculated from the phase derivatives between two successive refined pitch periods. In unvoiced speech the conventional amplitude spectrum is determined and for all harmonics the 'factor of noisiness' is set to maximum. In the synthesis part the phase of each harmonic is composed from an initial and a random value scaled by the 'factor of noisiness' as determined in the analysis. A technique of 'overlap' and 'add' of the inverse. Fourier transforms completes the synthesis. Our method improves speech synthesis quality audibly. Phases can be manipulated to achieve an optimum fit at diphone boundaries and large modifications in duration and pitch are possible without losing naturalness